Various process gases may be used in the manufacturing and processing of micro-electronics. In addition, a variety of chemicals may be used in other environments demanding high purity gases, e.g., critical processes, including without limitation microelectronics applications, wafer cleaning, wafer bonding, photolithography mask cleaning, atomic layer deposition, chemical vapor deposition, flat panel displays, disinfection of surfaces contaminated with bacteria, viruses and other biological agents, industrial parts cleaning, pharmaceutical manufacturing, production of nano-materials, power generation and control devices, fuel cells, power transmission devices, and other applications in which process control and purity are critical considerations. In those processes, it is necessary to deliver very specific amounts of certain process gases under tightly controlled temperature, pressure and flow rate conditions.
There are numerous process gases used in micro-electronics applications and other critical processes. One advantage of using process gases in micro-electronics applications and other critical processes, as opposed to liquid-based approaches, is that gases are able to access high aspect ratio features on a surface. For example, according to the International Technology Roadmap for Semiconductors (ITRS), current semiconductor processes should be compatible with a half-pitch as small as 20-22 nm. The next technology node for semiconductors is expected to have a half-pitch of 14-16 nm, and the FIRS calls for <10 nm half-pitch in the near future. At these dimensions, liquid-based chemical processing is not feasible, because the surface tension of the process liquid prevents it from accessing the bottom of deep holes or channels and the corners of high aspect ratio features. Therefore, process gases have been used in some instances to overcome certain limitations of liquid-based processes, because gases do not suffer from the same surface tension limitations.
Ozone is a gas that is typically used to clean the surface of semiconductors (e.g., photoresist stripping) and as an oxidizing agent (e.g., forming oxide or hydroxide layers). Plasma-based processes have also been employed to overcome certain limitations of liquid-based processes. However, ozone- and plasma-based processes present their own set of limitations, including, inter alia, cost of operation, insufficient process controls, undesired side reactions, and inefficient cleaning. More recently, hydrogen peroxide has been explored as a replacement for ozone in certain applications. However, for several reasons, hydrogen peroxide has been of limited utility. Highly concentrated hydrogen peroxide solutions present serious safety and handling concerns and obtaining high concentrations of hydrogen peroxide in the gas phase has not been possible using existing technology. Similar concerns have limited the feasibility of using other potentially beneficial process gases, such as hydrazine.
For additional reasons, gas phase delivery of process chemicals is preferred to liquid phase delivery. For applications requiring low mass flow for process chemicals, liquid delivery of process chemicals is not accurate or clean enough. Gaseous delivery would be desired from a standpoint of ease of delivery, accuracy and purity. Low vapor liquids, such as water and hydrogen peroxide, are generally not available in the gas phase and, thus, the gas phase must be created in situ from the corresponding liquid. One approach is to vaporize the process chemical component directly at or near the point of use. Vaporizing liquids provides a process that leaves heavy contaminants behind, thus purifying the process chemical. There is an approximately 1000-fold increase in volume when changing from the liquid to the gas phase. Gas flow devices are better attuned to precise control than liquid delivery devices. Additionally, micro-electronics applications and other critical processes typically have extensive gas handling systems that make gaseous delivery considerably easier than liquid delivery. However, for safety, handling, stability, and/or purity reasons, many process gases are not amenable to direct vaporization from their pure liquid phase.
Gas phase delivery of low volatility compounds presents a particularly unique set of problems. One approach is to provide a multi-component liquid source wherein the process chemical is mixed with a more volatile solvent, such as water or an organic solvent (e.g., isopropanol). This is particularly suitable for aqueous hydrogen peroxide or hydrazine solutions, as high concentrations of hydrogen peroxide or hydrazine present an explosion hazard. However, when a multi-component solution is the liquid source to be delivered (e.g., hydrogen peroxide and water), Raoult's Law for multi-component solutions becomes relevant, According to Raoult's Law, for an idealized two-component solution, the vapor pressure of the solution is equal to the weighted sum of the vapor pressures for a pure solution of each component, where the weights are the mole fractions of each component:Ptot=Paxa+Pbxb 
In the above equation, Ptot is the total vapor pressure of the two-component solution, Pa is the vapor pressure of a pure solution of component A, xa is the mole fraction of component A in the two-component solution, Pb is the vapor pressure of a pure solution of component B, and xb is the mole fraction of component B in the two-component solution. Therefore, the relative mole fraction of each component is different in the liquid phase than it is in the vapor phase above the liquid. Specifically, the more volatile component (i.e., the component with the higher vapor pressure) has a higher relative mole fraction in the gas phase than it has in the liquid phase. In addition, because the gas phase of a typical gas delivery device, such as a bubbler, is continuously being swept away by a carrier gas, the composition of the two-component liquid solution, and hence the gaseous head space above the liquid, is dynamic. Unless the more volatile component is continuously replenished, the mole fraction of the less volatile component will increase in the liquid over time.
Thus, according to Raoult's Law, if a vacuum is pulled on the head space of a multi-component liquid solution or if a traditional bubbler or vaporizer is used to deliver the solution in the gas phase, the more volatile component of the liquid solution will be preferentially removed from the solution as compared to the less volatile component. This limits the concentration of the less volatile component that can be delivered in the gas phase. For instance, if a carrier gas is bubbled through a 30% hydrogen peroxide/water solution, only about 295 ppm of hydrogen peroxide will be delivered, the remainder being all water vapor (about 20,000 ppm) and the carrier gas. For vapor pressure and vapor composition studies of various hydrogen peroxide solutions, see Hydrogen Peroxide, Walter C. Schumb, Charles N. Satterfield and Ralph L. Wentworth, Reinhold Publishing Corporation, 1955, New York, available at http://hdl.handle.net/2027/mdp.39015003708784.
The differential delivery rate that results when a multi-component liquid solution is used as the source of process gases prevents repeatable process control. Process recipes cannot be written around continuously changing mixtures. Controls for measuring a continuously changing ratio of the components of the liquid source are not readily available, and if available, they are costly and difficult to integrate into the process. In addition, certain solutions become hazardous if the relative ratio of the components of the liquid source changes. For example, hydrogen peroxide in water becomes explosive at concentrations over about 75%; and thus, delivering hydrogen peroxide by bubbling a dry gas through an aqueous hydrogen peroxide solution, or evacuating the head space above such solution, can take a safe solution (e.g., 30% H2O2/H2O) and convert it to a hazardous material that is over 75% hydrogen peroxide. Therefore, currently available delivery devices and methods are insufficient for consistently, precisely, and safely delivering controlled quantities of process gases in many micro-electronics applications and other critical processes.
For a variety of applications and processes, it would be advantageous to use gas phase processes based on chemicals that are more typically available as liquid solutions, e.g., organic and inorganic solvents, inorganic and organic acids and bases, and oxidizing agents and reducing agents. Example of such chemicals include, without limitation, hydrogen peroxide, hydrazine, or isopropanol. But the gas phase use of those chemicals has been limited by, inter alia, Raoult's Law, as well as safety, handling, and purity concerns. Therefore, a technique is needed to overcome these limitations and, specifically, to allow the use of gaseous process chemicals obtained from a multi-component liquid source.